The so-called Long Term Evolution (LTE) wireless communication networks developed by members of the 3rd-Generation Partnership Project (3GPP) use orthogonal frequency-division multiplexing (OFDM) in the downlink and Discrete Fourier Transform spread (DFT-spread) OFDM (also referred to as single-carrier frequency-division multiple access, or FDMA) in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. The uplink subframe has the same subcarrier spacing as the downlink, where the subcarrier spacing is the difference in frequency between the center of one subcarrier and the center of an immediately adjacent subcarrier. The uplink subframe has the same number of single-carrier FDMA (SC-FDMA) symbols in the time domain as OFDM symbols in the downlink—in other words, the symbol durations are the same for both the OFDM downlink and the SC-FDMA uplink.
In the time domain, LTE downlink transmissions are organized into radio frames of ten milliseconds, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 milliseconds, as shown in FIG. 2. For normal cyclic prefix, one subframe consists of fourteen OFDM symbols. The duration of each symbol, i.e., the symbol interval, is approximately 71.4 microseconds (μs).
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 milliseconds) in the time domain and twelve contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in time (1.0 milliseconds) is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Downlink transmissions are dynamically scheduled, in that in each subframe the base station transmits control information about which terminals data is transmitted to and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe. This number n=1, 2, 3 or 4 is known as the Control Format Indicator (CFI) and is broadcast by the base station, in the first OFDM symbol interval of each downlink subframe. The downlink subframe also contains common reference symbols, which are known to the receiver and used for coherent demodulation of the control information. A downlink system with CFI=3 OFDM symbols as control is illustrated in FIG. 3.
The reference symbols shown in FIG. 3 are the cell specific reference symbols (CRS), and are used to support multiple functions including fine time and frequency synchronization and channel estimation for certain transmission modes.
While the development and deployment of LTE networks provides users with greatly increased wireless data rates and has enabled the development of a wide variety of mobile broadband (MBB) services, demand for these services continues to grow. In addition to this increased demand for improved bandwidth and performance, new applications for special-purpose devices, such as machine-to-machine (M2M) devices, continue to be developed. These market forces indicate that a wireless communications technology with improved flexibility is needed, to better match the variety of service requirements for mobile data applications.